The present invention relates generally to current sensing devices for electrical systems, and more particularly to fault indicators for installation on electrical conductors which include means for concentrating the flux from the conductor to a predetermined sensing plane.
Various types of fault indicators have been constructed for detecting faults in electrical distribution systems, including clamp-on type fault indicators, which clamp directly over cables in the system, and test-point type fault indicators, which are mounted over test points in the cables or associated connectors. Such fault indicators may be either of the manually reset type, wherein it is necessary that the indicator be physically reset following each fault, or of the automatically reset type, wherein a fault indication is reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E. O. Schweitzer Mfg. Co. of Mundelien, Illinois, and in U.S. Pat. Nos. 3,676,740, 3,906,477, and 4,063,171 of the present inventor.
Detection of fault currents by self-resetting fault indicators is often most advantageously accomplished by means of a reed switch located within the indicator housing having contacts in close proximity to the conductor being monitored. Upon occurrence of an abnormally high fault-associated magnetic field around the conductor, the contacts close and actuate circuitry which magnetizes an internal pole piece to position to a trip position a target indicator visible from the exterior of the indicator. Upon restoration of current in the conductor another circuit may be actuated to reposition the target indicator to a reset non-fault indicating position.
The present invention is directed to improvements in fault indicators which provide for more reliable operation and improved sensitivity. Specifically, the present invention provides magnetic flux concentrating means in the form of magnetic pole pieces which form a low reluctance magnetic circuit between the circumference of the conductor, wherein the magnetic field of the conductor is concentrated, and the magnetic sensing plane of the indicator, within which the reed switch responds to an applied magnetic field.
Accordingly, it is a general object of the present invention to provide a new and improved fault indicator.
It is more specific object of the present invention to provide a new and improved fault indicator having improved reliability and sensitivity.